This invention relates, in general, to an optocoupling apparatus, and more particularly to a method for encasing an optocoupling apparatus.
Optocouplers (also referred to as optical couplers or optoisolators) have been used for electrical isolation in systems such as computers, power supplies, telecommunications, and controllers. Typically, optocouplers have comprised a light emitting diode electrically connected to one or more electrodes, and a light sensing diode connected to one or more additional electrodes.
The light emitting diode, the light sensing diode, and portions of the electrodes have been enclosed within a light coupling material. Further, the light coupling material and portions of the electrodes have been encased within an encapsulating material. The light coupling material typically has been a clear silicone gel that allows light to pass freely from the light emitting diode to the light sensing diode while maintaining electrical isolation between the electrodes.
What is more, the encapsulating material typically has been an opaque epoxy that reflects light from the light emitting diode back into the clear gel. A second function of the encapsulating material has been to prevent light originating from external sources, from reaching the clear gel. Further, the encapsulating material has served as a protective enclosure from external mechanical forces.
Although methods for encasing optocoupling devices have been developed, several inherent limitations do exist. One of the foremost problems with available encasing techniques has been the presence of a creepage path along the boundary surface between the light coupling material and the encapsulating material. This path introduces an electrical breakdown region between the two materials.
The breakdown phenomenon is further exacerbated by the difference in the coefficients of thermal expansion between the light coupling material and the encapsulating material. Typically, the coefficient of thermal expansion of the light coupling material is many times greater than that of the encapsulating material. During the cooling phase after encapsulation, the light coupling material will contract more than the encapsulating material, causing gaps to form at the interface between the two materials. These gaps comprise gas or air and have a lower dielectric strength than the light coupling material and the encapsulating material. Over time, moisture condenses along the electrodes to the boundary between the two materials and into the gaps, thereby significantly increasing the likelihood of electrical breakdown.
A method for increasing the breakdown voltage along the boundary surface was disclosed by Adams, et. al. in U.S. Pat. No. 4,645,551, which is hereby incorporated herein by reference. In this patent, Adams et. al. presented a method for improving the bonding between the light coupling material and the encapsulating material by irradiating the light coupling material with ultraviolet light. This process activates the light coupling material thereby promoting formation of covalent bonds between the light coupling material and the encapsulating material. Since the two materials are linked covalently creepage paths are eliminated, preventing voltage breakdown between electrodes along the interface of the two materials.
A further reliability issue brought about by unmatched coefficients of thermal expansion is the failure of interconnect electrodes. Typically, an interconnect electrode connects the emitter device with a portion of the leadframe and the detector device with a different portion of the leadframe. Moreover, a portion of these interconnect electrodes is encased within the light coupling material, and the remaining portion is surrounded by the encapsulating material. Again, the coefficients of thermal expansion between the light coupling and encapsulating materials have been different. Since the interconnect electrodes are in materials with different coefficients of thermal expansion, temperature cycling may cause stress or bond failures of the interconnect electrodes. Finally, the cost of encasing an optocoupler apparatus is a strong function of the material used in the encapsulation process. Since the encapsulating material surrounds the light coupling material, the encapsulating material must be capable of reflecting light. In addition the encapsulant has been used as a protective enclosure from mechanical forces. The cost of material with light reflective properties and mechanical strength is high. In addition, materials commonly used to promote light reflectivity are very destructive to mold assemblies. Accordingly, it would be beneficial to have a method for encasing an optocoupling apparatus that minimizes the costs of the materials, while improving upon boundary surface breakdown and failure due to thermal and mechanical stresses.